Coherent quantum emitters are a basic ingredient in many quantum information systems. Atom-like emitters in the solid state represent a promising platform that can be scalably integrated into nanophotonic devices. However, no single system has yet combined high brightness of narrowband emission and a low inhomogeneous distribution of photon frequencies from separate emitters (indistinguishability) with ease of incorporation into nanophotonic structures. For example, semiconductor quantum dots can be bright and simple to integrate into nanostructures, but have a notoriously large inhomogeneous distribution. Nitrogen-vacancy (NV−) centers in bulk diamond are bright and photostable, with a moderate inhomogeneous distribution that allows straightforward tuning of multiple NV− centers into resonance. These properties allow proof-of-principle demonstrations of quantum information protocols such as remote spin-spin entanglement generation and quantum teleportation. Further progress towards developing NV− based quantum devices has been hindered by low indistinguishable photon generation rates, a challenge that could be addressed by integrating NV− centers into nanophotonic structures. However, the optical transition frequencies of NV− centers are very sensitive to their local environment, making integration of spectrally stable emitters into nanophotonic structures a major challenge.
The negatively charged silicon-vacancy color center in diamond (SiV−) has shown promise in fulfilling the key criteria of high brightness, lifetime-limited optical linewidths, and a narrow inhomogeneous distribution of optical transition frequencies. The SiV− (FIG. 1) has electronic states with strong dipole transitions (excited state lifetime of under 1.7 ns) with 70% of the emission in the zero-phonon line (ZPL) at 737 nm. The inversion symmetry of the SiV− prevents first-order Stark shifts, suppressing spectral diffusion and allowing indistinguishable photons to be generated from separate emitters without the need for tuning or extensive pre-selection of emitters. When combined with a spin degree of freedom, the SiV− center's bright narrowband transition, narrow inhomogeneous distribution, and spectral stability make it a promising candidate for applications in quantum information science.
The negatively-charged germanium-vacancy color center in diamond (GeV−) is another promising color center with many of the same properties as the SiV−. It also has a bright, narrowband optical transition that is protected from Stark shifts and environmental inhomogeneities by inversion symmetry.
These color centers occur only rarely in nature, and are typically introduced during CVD growth via deliberate doping with silane (SiV− centers) or via Si/Ge contamination. While these techniques typically result in a narrow inhomogeneous distribution of color center fluorescence wavelengths, these samples have a number of disadvantages: the concentration of color centers is difficult to control, localization of color centers in three dimensions is impossible, and such samples cannot generally be obtained commercially.
Both the SiV and GeV color centers in diamond also exist in neutral charge states. The ground electronic states of these SiV0 and GeV0 color centers are believed to be orbital singlet states, leading to long spin coherence times even at room temperature. These neutral charge states are predicted to have high-quality optical properties again due to the inversion symmetry of the defect structure, and can also be created using ion implantation, either alone or with the addition of extra dopants or local gates to control the diamond Femi level.